For a method of producing a concavo-convex nonwoven fabric, a heat-compressed nonwoven fabric formed by using an embossing roll is widely known. However, since such a nonwoven fabric is formed by heat compression, the bulk of the thus obtained concavo-convex nonwoven fabric is rather low. The compressed part becomes like a film, and the feeling of the thus obtained nonwoven fabric deteriorates. Even the remaining parts other than the heat compressed part easily lose the bulkiness under the influence of the heat compressing action.
An example of methods for increasing the bulkiness is implied by a floor-cleaning sheet (see Patent document 1), which is produced by laminating a fibrous web based on a heat-adhesive fiber and a mesh sheet as a supporter, through which hot air is passed so as to integrate the fibrous web and the mesh sheet thereby forming concaves and convexes.
However, when the region of the fibrous web for passing the hot air is decreased due to the use of the mesh sheet, turbulence occurs in the hot air in a region where the hot air does not pass through. This causes some problems, for example, the accumulation of fibers in the web is disordered. Adhesiveness is degraded at parts not passing the hot air, and the nonwoven fabric strength deteriorates. Thus the shape of the concave-convex and the area of the concaves are restricted.
In an alternative method disclosed for forming a concavo-convex nonwoven fabric, a nonwoven fabric is prepared by partially heat-compressing to join two layers by use of a heat-embossing roll, where the first layer containing heat shrinkable fibers has a maximal heat-shrinkage developing temperature that is lower than the melting point of a second layer made of non-heat shrinkable fibers, and by heating the nonwoven fabric to shrink the heat shrinkable layer so as to form the concavo-convex nonwoven fabric (see Patent document 2).
In this case, the concave parts become film-like to some extent, and thus the air permeability and the feeling deteriorate when the concave area is increased. The feeling becomes rigid when the temperature at the heating for shrinkage is higher than the melting point of the unshrinkable fiber. On the other hand, when the temperature in heating is equal or lower than the melting point of the unshrinkable fibers, the feeling is improved, but problems occur, for example, the sufficient strength cannot be obtained, and the dimensional stability of the obtained nonwoven fabric is not satisfactory. Since the convex parts have a lot of voids, the resistance against stress is inherently insufficient.